The present invention relates to improvements in method and apparatus for assembling universal joints.
It is already known from West German Publication No. 1,527,557 that bearings can be pressed into the bores of the fork arms of a universal joint assembly by means of floating pressure units without any reaction forces and fixed axially in place, for example, by peening. By this arrangement, the joint cross is positioned in a peening operation in a finished mounting arrangement. The cross piece of the joint is supposed to be free of axial play and to produce in the pivoting direction a braking moment which is often referenced as a bending moment. The bending moment is produced by reason of the fact that the bearings are fixed in place while the fork arms are spread apart under pretension whereby the lower face of the bearing cups lie against the ends of the universal joint pins. In this known arrangement, the bearings are first pressed into the bearing bores simultaneously, whereby each fork arm is supported by a hook-like support element from the rear to secure it to each of the pressure units. In this manner, the pressure can act on the bearings by way of pressing tools without creating reaction forces. As soon as the bearings in the known system are in contact with the pins of the joint cross, continued pressing of the bearing spreads the fork arms which results in outward displacement of the floating pressure units. The displacement distance is limited by adjustable stops whereby a pretension is obtained by way of the spreading distance of the fork arms, which can be determined in advance.
These prior apparatus and systems have several disadvantages and drawbacks. Since manufacturing tolerances can cause cross-sectional and/or structural differences between the two fork arms, the joint fork being in cross section an unworked casting, there are differences in the restoring forces or pretensions of the fork arms even though the fork arms are pushed outward to the same extent. The differences in the restoring forces act by way of the bearings directly on the joint cross and shift it out of the precentered position. This produces non-tolerable misalignment of the centers of the two joint parts with respect to each other which can no longer be optimally compensated even by individual balancing, since the joint parts must be bent toward each other in order to fulfill their function. The difference between the restoring forces in the two fork arms also means that the bending moment varies sharply from one joint to another. This defect is aggravated even further by the unavoidable tolerances of the joint forks relative to one another and not merely between the fork arms of one joint fork.
A further disadvantage of these prior known designs becomes evident when there are different fit tolerances between the bearing and bearing bore in the two fork arms. This causes different pressing pressures which, particularly in the spreading phase, because of the internal stress results in pretension which differs considerably from the adjusted spreading distance so that the center misalignment of the joint parts can become even greater.
An additional center misalignment can also be anticipated in the case of the known design, because even if the machine stand is designed to be extremely stable, the differences in the pressing forces result in unilateral, elastic resilience. The joint parts themselves account for most of this difference because of a lack of internal stiffness. Further, for space considerations there is also the need for arranging the devices for clamping the joint parts during assembly at a distance from the point at which the force is actually applied. When the bearings are pressed into place, it cannot be avoided, therefore, that the bearing, which is easier to press in, reaches the pin of the joint cross first and because of the pressing force initially places the pin under pressure in a unilateral manner with respect to the joint fork. Depending on the stiffness, this causes a center misalignment which can no longer be compensated even after the other bearing has been seated against the joint cross, because normally, in order to avoid other defects, both pressure units exert equal forces once the bearings have made contact with the joint cross. The joint cross can, therefore, no longer leave the misalignment position it has assumed with respect to the joint fork. The same problem exists in the case of different peening forces. The above difficulties cannot be avoided except by precise individual pairing and matching of parts for each fork arm before each assembly operation. Thus, even though good results can be obtained with prior known apparatus and methods, they can only be obtained under ideal conditions, i.e. when the fork arms are completely free of tolerances as between themselves and from joint fork to joint fork, and correspondingly tolerance-free bearings are also available. As is commonly known, this is impossible in every day mass production assembly of parts of this type.